[3.13] gh-126312: Don't traverse frozen objects on the free-threaded build (GH-126338...

[3.13] gh-126312: Don't traverse frozen objects on the free-threaded build (GH-126338) (#126866)

* Fix merge conflicts.

* [3.13] gh-126312: Don't traverse frozen objects on the free-threaded build (GH-126338)

Also, _PyGC_Freeze() no longer freezes unreachable objects.

(cherry picked from commit d4c72fed8cba8e15ab7bb6c30a92bc9f2c8f0a2c)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
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Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>